THS7376 Datasheet, PDF (49/61 Pages) Texas Instruments – 4-Channel Video Amplifier with One SD and Three HD 8th-Order Filters with 6-dB Gain

THS7376
www.ti.com
SBOS692 – JUNE 2013
Another concern about passive filters is the use of inductors. Inductors are magnetic components, and are
therefore susceptible to electromagnetic coupling and interference (EMC/EMI). Coupling can occur because of
other video channels nearby using inductors for filtering, or coupling can come from nearby switched-mode
power supplies. Some other forms of coupling could be from outside sources with strong EMI radiation and can
cause failure in EMC testing such as required for CE compliance.
One concern about an active filter in an integrated circuit is the variation of the filter characteristics when the
ambient temperature and the subsequent die temperature changes. To minimize temperature effects, the
THS7376 uses low-temperature coefficient resistors and high-quality, low-temperature coefficient capacitors
found in the BiCom3X process. These filters are specified by design to account for process variations and
temperature variations to maintain proper filter characteristics. This approach maintains a low channel-to-channel
time delay that is required for proper video signal performance.
Another benefit of the THS7376 over a passive RLC filter is the input and output impedance. The input
impedance presented to the DAC from passive filters varies significantly, from 35 Ω to over 1.5 kΩ, and may
cause voltage variations over frequency. The THS7376 input impedance is 800 kΩ, and only the 2-pF input
capacitance plus the PCB trace capacitance impacts the input impedance. As such, the voltage variation
appearing at the DAC output is better controlled with a fixed termination resistor and the high input impedance
buffer of the THS7376.
On the output side of the filter, a passive filter again has a large impedance variation over frequency. The
EIA/CEA-770 specifications require the return loss to be at least 25 dB over the video frequency range of usage.
For a video system, this requirement implies the source impedance (which includes the source, series resistor,
and filter) must be better than 75 Ω, ±9 Ω. The THS7376 is an operational amplifier that approximates an ideal
voltage source, which is desirable because the output impedance is very low and can source and sink current.
To properly match the transmission line characteristic impedance of a video line, a 75-Ω series resistor is placed
on the output. To minimize reflections and to maintain a good return loss meeting EIA/CEA specifications, this
output impedance must maintain a 75-Ω impedance. A wide impedance variation of a passive filter cannot
ensure this level of performance. On the other hand, the THS7376 has approximately 0.7 Ω of output impedance
(or a return loss of 46 dB, at 6.75 MHz for the SD filter) and approximately 1.3 Ω of output impedance (or a
return loss of 41 dB, at 30 MHz for the HD filters). Thus, the system is matched significantly better with a
THS7376 compared to a passive filter.
One final benefit of the THS7376 over a passive filter is power dissipation. A DAC driving a video line must be
able to drive a 37.5-Ω load: the receiver 75-Ω resistor and the 75-Ω impedance matching resistor next to the
DAC to maintain the source impedance requirement. This requirement forces the DAC to drive at least 1.25 V_P
(100% saturation CVBS) / 37.5 Ω = 33.3 mA. A DAC is a current-steering element, and this amount of current
flows internally to the DAC even if the output is 0 V. Thus, power dissipation in the DAC may be very high,
especially when four channels are being driven. Using the THS7376 with a high input impedance and the
capability to drive up to two video lines per channel can reduce DAC power dissipation significantly. This
outcome is possible because the resistance that the DAC drives can be substantially increased. Setting this
resistance in a DAC by a current-setting resistor on the DAC itself is a common practice. Thus, the resistance
can be 300 Ω or more, substantially reducing the current drive demands from the DAC and saving significant
amounts of power. For example, a 3.3-V, four-channel DAC dissipates 440 mW alone for the steering current
capability (four channels × 33.3 mA × 3.3 V) if the DAC must drive a 37.5-Ω load. With a 300-Ω load, the DAC
power dissipation as a result of current steering current would only be 55 mW (four channels × 4.16 mA × 3.3 V).
Copyright © 2013, Texas Instruments Incorporated
Product Folder Links: THS7376
Submit Documentation Feedback
49